Equalization is an error correction technique to correct for loss caused by cables, wires or other signal transmission media used for communication of serial data. For example, equalization may correct for distortion caused by intersymbol interference (ISI) of high data rate signals within transmission cables connecting various transmitters and receivers within a computer system, such as a mother board chipset, central processing unit (CPU), hard disk drive, memories, peripheral devices, and other components capable of sending and/or receiving data signals.
Feed forward equalization (FFE) is a type of error correction that applies a linear filter to a received signal, and may be implemented as a hardware or software filter. For example, test equipment manufacturers implement FFE filters using a digital finite impulse response (FIR) filter to emulate the equalized signal received by a receiving device in a computer system. Equalizers are characterized by a finite number of equalization coefficients. To be effective, these coefficients need to be optimized to correct for the specific ISI induced upon the transmitted signal by the transmission media.
Determining the optimal equalization coefficients by analyzing distorted signals that have been transmitted from a transmitter to a receiver through the transmission media is called training. Conventional training algorithms require knowledge of the specific bit sequence transmitted during the training process. Normally, the ISI is so severe that the receiver cannot determine the training sequence without first applying the proper equalization. Therefore, the transmitter and receiver use a predetermined periodic bit sequence (i.e., a training pattern), transmitted during the training process, in order to determine equalization coefficients. A cross correlation function may then be used to determine the exact phase of the transmitted bit sequence in the received signal. However, use of a predetermined training pattern increases complexly in the training process, particularly in the case where electronic test equipment is used to emulate the equalization of an actual receiver e.g., within a device under test (DUT).